Introduction to ENVIRONMENTAL TOXICOLOGY Impacts of Chemicals Upon Ecological Systems - CHAPTER 8

CHAPTER 8 Heavy Metals Pollution caused by heavy metals is now a worldwide phenomenon. Among the many heavy metals, lead (Pb), mercury (Hg), cadmium (Cd), arsenic (As), chromium (Cr), zinc (Zn), and copper (Cu) are of most concern, although the last three metals are essential nutrients in animal and human nutrition. These metals are widely used in industry, particularly in metal-working or metal-plating, and in such products as batteries and electronics. They also are used in the production of jewelry, paint pigments, pottery glazes, inks, dyes, rubber, plastics, pesticides, and even in medi-cines. These metals enter the environment wherever they are produced, used, and ultimately discarded. Heavy metals are very toxic because, as ions or in compound forms, they are soluble in water and may be readily absorbed into living organisms. After absorption, these metals can bind to vital cellular components such as structural proteins, enzymes, and nucleic acids, and interfere with their functioning. In humans, some of these metals, even in small amounts, can cause severe physiological and health effects. In this chapter, we will consider Pb, Cd, and Hg, the three heavy metals widely recognized as the most toxic in our environment. LEAD Lead (Pb) is one of the ancient metals and has been used by humans for several thousands of years. Pb plays an important role in the economy of all industrialized countries in the world. In the U.S., the industrial consumption of Pb is estimated to be about 1.3 million tons per year, with a concomitant annual emission of about 600,000 tons of Pb into the environment (NAS 1980). Additional amounts are added through mining, smelting, manufacturing, disposal, and recycling processes. Fur-thermore, until recently huge amounts of Pb and its compounds had been emitted into the atmosphere as a result of leaded gasoline combustion. Consequently, Pb is ubiquitous in our environment. Because Pb is toxic to humans at high doses, levels of exposure encountered by some members of the population constitute a serious public health problem (NAS). The importance of Pb as an environmental pollutant is apparent since the Environ-mental Protection Agency (EPA) has designated Pb as one of the six criteria air pollutants. Properties and Uses Lead has a low melting point (326°C). It is a soft, malleable metal and it can be easily formed into a variety of shapes. It also can form alloys with many other metals. Other important industrial products containing Pb include pipes, paints, solders, glass, pottery glazes, rubber, plastics, and insecticides. Exposure Atmospheric Lead Sources of atmospheric Pb include lead smelters, burning of coal and materials containing Pb, refining of scrap, wind blown from soils, and lead alkyls from gasoline. Effluents from smokestacks and other gaseous emissions from smelters and refining processes can distribute significant quantities of Pb into the air, soils, and vegetation growing nearby. However, the most common source of Pb contam-ination in ambient air until recently was the exhaust from automobiles. Tetraethyl lead was introduced as an antiknock agent in gasoline in the 1920s and since then has played an increasingly important role as an atmospheric pollutant. Following the mandatory use of unleaded gasoline and improved industrial emission control, atmospheric Pb emission has decreased dramatically. According to an EPA report, Pb emission from major emission sources in the U.S. decreased from 56,000 to 7100 metric tons per year between 1981 and 1990 (EPA 1991). While the atmospheric Pb pollution problem in other developed countries likewise has been significantly reduced, a similar trend has not occurred in many third-world countries. Waterborne Lead Surface waters may contain significant amounts of Pb when subjected to some special contamination. About 14% of representative drinking water supplies (i.e., piped drinking water) were found to contain more than 10 mg/l in a 1963–1965 survey. Less than 1% was found to be in excess of 30 mg/l. On the other hand, rainwater collected near a busy highway may contain as much as 50 mg/l. Another serious problem related to waterborne Pb is lead shot left in the North America’s lakes and ponds. A large number of waterfowl in the U.S. are poisoned or killed following ingestion of the shot. Lead in Food Food has long been a major source of Pb intake for animals and humans. Animals may ingest Pb-contaminated vegetation and become intoxicated. In humans, Pb may be ingested through Pb-contaminated containers or Pb pottery glazes. Researchers suggest that some Roman emperors might have become ill and even died from Pb poisoning by drinking wines contaminated with high levels of Pb. Vegetation growing near highways has been shown to accumulate high amounts of Pb deposited from automobile exhaust (Lagerwerff et al. 1973; Khalid et al. 1996). Pica, children’s craving for unnatural foods, is thought responsible for the chronic Pb poisoning among many poor urban children, as they eat flaking paint from the walls of old houses. About 27 million housing units were built before 1940 when Pb was in common use (Lin-Fu 1982). Lead paint poses a major threat for children and is one of the major public health problems that many communities face. Lead in Soils Lead and other metals can impact soils and biota by deposition from polluted air. Stack emission from smelters (Little and Martin 1972) and emission from automobile exhaust systems along highways are examples. Pb contamination due to mine wastes also is an important problem in areas surrounding metal mines. Earlier reports indicate that about 50% of the Pb liberated from motor vehicles in the U.S. was deposited within 30 m of the roadways (Ryan 1976) and the remainder was scattered over large areas. Lead accumulation in soils near roads varies with traffic volume and decreases rapidly with distance from the road. For example, Pb con-centrations of 128 to 700 ppm are found in soil adjacent to 12 highways in the Minneapolis-St. Paul area (Ryan 1976). These levels were much greater than the reported value of 10 to 15 ppm in unpolluted rural soils. Grass collected near an intersection of two heavily traveled highways near Denver, CO contained as much as 3000 ppm Pb, while vegetable samples from gardens less than 50 ft from roads in Canandaigua, NY averaged 115 ppm Pb (range: < 10 ppm to 700 ppm). In an attempt to assess the effect of the mandatory use of unleaded gasoline in new automobiles on Pb concentrations in highway soils, Byrd et al. (1983) studied Pb concentrations in soils along U.S. Interstate 20 in northeast Louisiana and observed that the concentrations increased from 1973 to 1974 but decreased from 1975 to 1979. They concluded that the mandatory use of unleaded gasoline had significantly reduced the Pb concentrations in soils near highways. Lead Toxicity Effect on Plants Plants exposed to high levels of Pb from ambient air and soils can accumulate the metal and manifest toxicity. The toxicity and presence of other trace metals vary greatly among plant species. Based on in vitro studies, toxicity sequences have been determined for several species. Barley plants were shown to be more sensitive to Pb than to Cr, Cd, Ni, or Zn (Oberlander and Roth 1978), and exposure to relatively high levels of Pb was shown to inhibit seed germination (Koeppe 1977; Yu 1991). The effect of Pb on germination, however, was found to be less severe compared to several other metals such as Cd, As, and Hg (Koeppe; Fargasova 1994). It is important to note that, following plant uptake, Pb moves into the food chain and thus can affect animals and humans. Effect on Animals The effect of Pb on freshwater fish varies depending on the species of fish. Goldfish, for example, are relatively resistant to Pb, presumably due to their abundant gill secretion. As mentioned above, following the ingestion of expended lead shot in lakes or in the field, more than one million birds are estimated killed each year in the U.S. Lead absorbed by the bird paralyzes the gizzard leading to starvation, and death usually follows within several weeks after the exposure. Effect on Humans Daily intake of Pb in humans is estimated to range from 20 mg to 400 mg per person. The FAO/WHO Expert Committee established a Provisional Tolerable Weekly Intake (PTWI) of 3000 mg, approximately 500 mg/day. Only one-half of this amount appears to be safe for children. About 5 to 15% of ingested Pb is absorbed. This amounts to 15 to 25 mg/day and represents two-thirds of the total absorbed Pb. By contrast, about 20 to 40% of the inhaled Pb is absorbed, amounting to about 8 mg/day, or one-third of the total absorbed Pb. The considerably higher blood Pb levels in industrial populations reflect wide-spread environmental Pb pollution. However, data obtained from the Second National Health and Nutrition Examination Survey (NHANES II) indicate that there has been a reduction in the overall mean blood-lead level of the U.S. population during the period 1976 through 1980, from 15.8 mg/dl to 10.0 mg/dl (Lin-Fu 1982). It is suggested that an increased use of unleaded gasoline by the U.S. population may be responsible for the observed decrease. Lead is one of the systemic poisons in that once absorbed into the circulatory system, it is distributed throughout the body where it causes serious health effects. Manifested effects of Pb poisoning include nausea, anorexia, severe abdominal cramps, weight loss, anemia, renal tubular dysfunction, muscle aches, and joint pains. Lead can pass the placental barrier and may reach the fetus, resulting in miscarriages, abortions, and stillbirths. Through interaction with cellular components of brain cells, Pb also adversely affects the central nervous system (CNS). Clinical symptoms such as encephalopa-thy, convulsions, and delirium may occur. In severe cases coma and death may follow. These injuries are often reflected by behavioral disturbances observed in Pb-poisoned victims. It is estimated that approximately 90% of Pb absorbed by humans is deposited in the bone (Aufderheide and Wittmers 1992). Bone, however, is no longer consid-ered a sink for Pb in the body. Rather, it is recognized as a two-way process of active influx and efflux of Pb to and from the bone and blood stream (Silbergeld et al. 1993). As a result, bone acts like a reservoir for Pb, thus influencing the exposure of the metal in the body. Although there is evidence that both inorganic and organic lead compounds are carcinogenic in experimental animals (Cherlewski 1979; Blake and Mann 1983), no conclusive evidence has been reported in humans. Biochemical Effect Lead is taken up and transported in plants (Cannon and Bowles 1962) and can decrease cell division at very low concentrations. Lead inhibits the electron transport in corn mitochondria, especially when phosphate is present (Koeppe and Miller 1970). Lead, as mentioned above, is a systemic poison and can induce deleterious effects in living organisms. The biochemical effect of Pb is complex and, in certain areas, its mode of action remains unclear. Several well-established biochemical effects are discussed here. First, as an electropositive metal, Pb has a high affinity for the sulfhydryl (SH) group. Enzymes that depend on the SH group as the active site are, therefore, inhibited by Pb. In this case, Pb reacts with the SH group on the enzyme molecule to form mercaptide, leading to inactivation of the enzyme. The following reaction depicts such a relationship: 2RSH + Pb2+ ® R–S–Pb–S–R + 2H+ Examples of the sulfhydryl-dependent enzymes include adenyl cyclase and aminotransferases. Adenyl cyclase catalyzes the conversion of ATP to cyclic AMP needed in brain neurotransmission. Aminotransferases are involved in transamination and thus important in amino acid metabolism. Second, divalent Pb is similar in many aspects to Ca and may exert a competitive action on body processes such as mitochondrial respiration and neurological func-tions. Lead can compete with Ca for entry at the presynaptic receptor. Since Ca evokes the release of acetylcholine across the synapse, this inhibition manifests itself in the form of decreased endplate potential. The miniature endplate potential release of subthreshold levels of acetylcholine has been shown to be increased (Barton et al. 1978). The close chemical similarity between Pb and Ca may partially account for the fact that they seem interchangeable in biological systems and that 90% or more of the total body burden of Pb is found in the skeleton. Third, Pb can interact with nucleic acids, leading to either decreased or increased protein synthesis. Lead has been shown to reduce the ability of t-RNA to bind ribosomes. The effect of Pb on nucleic acids, therefore, has important biological implications (Barton et al. 1978). Finally, it is widely known that Pb impairs the formation of red blood cells. The mechanism involved in the impairment is that Pb inhibits both d-aminolevulinic acid dehydratase (ALA-D)(Hernberg et al. 1970) and ferrochelatase (Tephly et al. 1978). These are two key enzymes involved in heme biosynthesis. ALA-D catalyzes the conversion of d-aminolevulinic acid into porphobilinogen (PBG), whereas ferroche-latase is responsible for catalyzing the incorporation of Fe2+ into protoporphyrin IX to form heme (Figure. 8.1). Lead inhibition of the two enzymes appears to be due to its interaction with Zn and Fe required in the process. ... - tailieumienphi.vn